1. Field of the Invention The present invention relates to a wideband capable directional coupler.
2. Description of the Related Art
Directional couplers are used for detecting the levels of transmission/reception signals in transmission/reception circuits of wireless communication apparatuses such as cellular phones and wireless LAN communication apparatuses.
A directional coupler configured as follows is known as a conventional directional coupler. The directional coupler has an input port, an output port, a coupling port, a terminal port, a main line, and a subline. One end of the main line is connected to the input port, and the other end of the main line is connected to the output port. One end of the subline is connected to the coupling port, and the other end of the subline is connected to the terminal port. The main line and the subline are configured to be electromagnetically coupled to each other. The terminal port is grounded via a terminator having a resistance of, for example, 50 Ω. The input port receives a high frequency signal, and the output port outputs the same. The coupling port outputs a coupling signal having a power that depends on the power of the high frequency signal received at the input port.
Major parameters indicating the characteristics of directional couplers include insertion loss, coupling, isolation, directivity, and return loss at the coupling port. Definitions of these parameters will now be described. First, assume that the input port receives a high frequency signal of power P1. In this case, let P2 be the power of the signal output from the output port, P3 be the power of the signal output from the coupling port, and P4 be the power of the signal output from the terminal port. Assuming that the output port receives a high frequency signal of power P02, let P03 be the power of the signal output from the coupling port. Assuming that the coupling port receives a high frequency signal of power P5, let P6 be the power of the signal reflected at the coupling port. Further, let IL represent insertion loss, C represent coupling, I represent isolation, D represent directivity, and RL represent return loss at the coupling port. These parameters are defined by the following equations.IL=10 log (P2/P1) [db]C=10 log (P3/P1) [db]I=10 log (P03/P02) [dB]D=10 log (P4/P3) [dB]RL=10 log (P6/P5) [dB]
The coupling of the conventional directional coupler increases with increasing frequency of the high frequency signal received at the input port. The conventional directional coupler thus suffers from the problem that the frequency response of the coupling is not flat. Where coupling is denoted as −c (dB), an increase in coupling means a decrease in the value of c.
U.S. Pat. No. 9,077,061 B2 discloses a directional coupler aiming to resolve the aforementioned problem. The directional coupler disclosed therein has a subline divided into a first subline and a second subline. One end of the first subline is connected to the coupling port. One end of the second subline is connected to the terminal port. A phase conversion unit is provided between the other end of the first subline and the other end of the second subline. The phase conversion unit causes a phase shift to be generated in a signal passing therethrough in such a manner that the absolute value of the phase shift monotonically increases within the range from 0 degree to 180 degrees as the frequency increases in a predetermined frequency band. The phase conversion unit is specifically a low-pass filter.
Mobile communication systems conforming to the Long Term Evolution (LTE) standard have become practically used in recent years, and further, practical use of mobile communication systems conforming to the LTE-Advanced standard, which is an evolution of the LTE standard, is under study. Carrier Aggregation (CA) is one of the key technologies of the LTE-Advanced standard. CA uses multiple carriers called component carriers simultaneously to enable wideband transmission.
A mobile communication apparatus operable under CA uses multiple frequency bands simultaneously. Accordingly, such a mobile communication apparatus requires a wideband capable directional coupler, that is, a directional coupler usable for multiple signals in multiple frequency bands.
Directional couplers for use in wireless communication apparatuses are sometimes expected to be bidirectional, that is, to exhibit the same characteristics even when used with the input port and the output port reversed and with the coupling port and the terminal port reversed, as compared with when used without reversing them. Examples of situations requiring bidirectionality include when a directional coupler provided in a transmission circuit for supplying a transmission signal to an antenna is used to detect the level of the transmission signal and also the level of a reflected wave signal resulting from the transmission signal reflected at the antenna. The detection of the level of the reflected wave signal by the directional coupler is for the purpose of adjusting the characteristics of an impedance matching element provided between the transmission circuit and the antenna so as to sufficiently reduce the level of the reflected wave signal. In the aforementioned example, when the directional coupler detects the level of a transmission signal, the transmission signal is received at the input port and output from the output port, and the coupling port outputs a signal having a power that depends on the level of the transmission signal. On the other hand, when the directional coupler detects the level of a reflected wave signal, the reflected wave signal is received at the output port and output from the input port, and the terminal port outputs a signal having a power that depends on the level of the reflected wave signal.
JP 2014-057207A discloses a directional coupler that is wideband capable and bidirectional. In the directional coupler disclosed in JP 2014-057207A, the subline includes a first coupling section having strong coupling to the main line, a second coupling section having weak coupling to the main line and located closer to the coupling port than the first coupling section, a third coupling section having weak coupling to the main line and located closer to the isolation port (terminal port) than the first coupling section, a first non-coupling section not coupled to the main line, extending between the first and second coupling sections and having a length of a quarter or more of a wavelength corresponding to the service frequency band, and a second non-coupling section not coupled to the main line, extending between the first and third coupling sections and having a length of a quarter or more of the wavelength corresponding to the service frequency band.
The directional coupler disclosed in U.S. Pat. No. 9,077,061 B2 has insufficient isolation in a frequency band not lower than the cut-off frequency of the low-pass filter. More specifically, where isolation is denoted as −i (dB), this directional coupler does not exhibit a sufficiently large value of i in a frequency band not lower than the cut-off frequency of the low-pass filter. Thus, this directional coupler does not work in a frequency band not lower than the cut-off frequency of the low-pass filter.
We proceed to explain why the directional coupler disclosed in U.S. Pat. No. 9,077,061 B2 does not exhibit sufficiently large value of i in a frequency band not lower than the cut-off frequency of the low-pass filter. In this directional coupler, there are formed a path connecting the connection point between the first subline and the low-pass filter to the ground via only a first capacitor, and a path connecting the connection point between the second subline and the low-pass filter to the ground via only a second capacitor. Consequently, in a frequency band not lower than the cut-off frequency of the low-pass filter, a high frequency signal going from the first sublime to the low-pass filter mostly flows to the ground via the first capacitor, and a high frequency signal going from the second subline to the low-pass filter mostly flows to the ground via the second capacitor. Thus, in this directional coupler, most part of the high frequency signal fails to pass through the low-pass filter in a frequency band not lower than the cut-off frequency of the low-pass filter.
For the reason described above, the directional coupler disclosed in U.S. Pat. No. 9,077,061 B2 is only usable over a limited frequency band lower than the cut-off frequency of the low-pass filter. Providing a wideband capable directional coupler is thus difficult with the technology described in U.S. Pat. No. 9,077,061 B2.
In the directional coupler disclosed in JP 2014-057207A, the subline includes the first and second non-coupling sections each having a length of a quarter or more of the wavelength corresponding to the service frequency band. The two non-coupling sections are each extremely long. For example, if the service frequency is 3 GHz, then the wavelength is 10 cm and the length of each of the two coupling sections is 2.5 cm or more. In JP 2014-057207A, each of the two non-coupling sections is formed of a long and narrow spiral line. Because of the two extremely long non-coupling sections, the directional coupler disclosed in JP 2014-057207A is disadvantageously large in footprint.